Brushless Direct Current (BLDC) motors are a type of electrical motor having multiple status phases (e.g. three or more) and a permanent magnet as a rotor (e.g. a bar magnet or a multi-pole magnet). BLDC motors do not use brushes for commutation, but instead they are electronically commutated. This implies, however, that the drive circuit must know the relative position of the rotor with respect to the stator, in order to direct the magnetic field for exerting torque on the rotor.
One kind of BLDC motors uses sensor devices such as e.g. optical sensor devices or Hall elements positioned around the rotor for determining the relative position using a resolver. This, however, has the disadvantage of increased complexity, extra component cost and decreased reliability, especially in harsh motor environments.
Another kind of BLDC motors does not use such sensors, but uses the back-EMF (BEMF) generated in the windings for determining the rotor position. The shape of BEMF can e.g. be trapezoidal or sinusoidal, depending on how the windings are located on the stator. The working principles of synchronous brushless DC motors based on back-EMF are well known and well described in the art, for example in U.S. Pat. No. 4,275,343 (filed in 1978, almost 35 years ago) and U.S. Pat. No. 4,455,513 (filed in 1982) and many others. Several of these other applications are based on integration of the back-EMF signal, or on the zero-crossing of the back-EMF signal for determining the relative rotor position. A disadvantage of such methods is that they require additional and often complex circuitry, such as e.g. integrators, comparators, filters, etc.
In U.S. Pat. No. 7,737,651B2, De Four discloses a principle for determining the rotor position based on space vector theory. Besides from being highly theoretical and complex, the execution of this algorithm requires a powerful DSP.
US20030062860 describes a control system operating with a PWM speed regulation of a sensorless BLDC motor with a plurality of windings, and a hybrid method utilizing BEMF induced in stator windings to indicate the commutation instant and a falling edge detection method to override PWM chopping and commutation noises. The circuit and software are alleged to be relatively concise and low cost. Yet, the control device includes an analog filter and a comparator for generating the BEMF zero crossing points (ZCP). Other functions such as detecting the falling edge of the generated ZCPs may be implemented in an integrated controller.